1. Field of the Invention
The present invention relates to an apparatus for heating polycrystalline silicon used for the production of semiconductor grade polycrystalline silicon by the fluidized-layer method, the heat treatment of semiconductor grade polycrystalline silicon produced by the bell-jar method and the fluidized-layer method and the like.
2. Discussion of the Background
The typical conventional method of producing semiconductor grade polycrystalline silicon is the bell-jar method. In this method, a silicon rod put in a bell-jar type reaction vessel is electrified and thereby heated and a gas mixture of semiconductor grade trichlorosilane and semiconductor grade hydrogen or monosilane and semiconductor grade hydrogen is introduced into the reaction vessel under conditions sufficient to deposit semiconductor grade silicon on a surface of the heated silicon rod.
In addition, the production of polycrystalline silicon by the fluidized-layer method has also recently been started. In the production of polycrystalline silicon by the fluidized-layer method, small pieces of polycrystalline silicon having particle sizes of 0.1 to 3 mm are charged into a reaction vessel and a reaction gas comprising trichlorosilane or monosilane and the like of semiconductor grade is introduced into the reaction vessel while heating the reaction vessel by means of an outside heater. The small pieces of silicon within the reaction vessel are fluidized and silicon of semiconductor grade is deposited on the surface of the small pieces of silicon.
Polycrystalline silicon produced by the bell-jar method and the fluidized-layer method is used as a material for production of, for example, single-crystalline silicon. However, in the case where polycrystalline silicon is turned into single-crystalline silicon by the Chokralskii method, the polycrystalline silicon bursts and scatters when heated up to its melting point. It is said that this bursting phenomenon occurs due to thermal strain resulting from the high-speed heating of polycrystalline silicon or expansion of gases remaining in the product. In order to prevent this bursting phenomenon, polycrystalline silicon of semiconductor grade produced by the bell-jar method and the fluidized layer method has been subjected to a heat treatment at 600.degree. to 1,400.degree. C. to remove the thermal strain and remaining gases.
In this heat treatment, semiconductor grade polycrystalline silicon within the reaction vessel is heated by means of an outside heater while passing an inert gas through the reaction vessel. Then, the polycrystalline silicon is subjected to the heat treatment at 600.degree. to 1,400.degree. C., whereby the thermal strain and the remained gases are removed
Here, semiconductor grade indicates concentrations of impurities such that [B]&lt;0.5 ppba, [P]&lt;0.5 ppba, [C]&lt;0.4 ppma, and a total concentration of heavy metals &lt;30 ppba. These concentrations are judged as suitable for the starting material for single-crystalline semiconductor grade silicon produced by the Chokralskii method.
However, comparing the bell-jar method with the fluidized-layer method of producing polycrystalline silicon, the former is a batch type process and requires the assembly and disassembly of the vessel while the latter is a continuous type process and does not require the assembly and disassembly. In addition, in the former, the reaction vessel is cooled but in the latter the reaction vessel is not cooled. Furthermore, the reaction surface area of the latter is larger than that of the former. In view of the above described characteristics, production by the fluidized-layer method is more efficient and requires a reduced consumption of electric power in comparison with the bell-jar method.
However, in the production of polycrystalline silicon by this fluidized-layer method, the reaction vessel is directly heated and small pieces of silicon are brought into direct contact with the wall of the reaction vessel, so that contamination of silicon due to the wall of the reaction vessel becomes a problem. A similar problem also occurs in the heat treatment of polycrystalline silicon in which silicon is brought into direct contact with the wall of the reaction vessel. This problem arises from the fact that the conventional reaction vessel is made of graphite, quartz, SiC and the like.
That is to say, in the reaction vessel made of graphite, polycrystalline silicon is brought into contact with the graphite wall when heated, so that carbon (C) enters the polycrystalline silicon from the graphite wall to contaminate the polycrystalline silicon with carbon. At the same time, since the porosity of graphite is large, there is the possibility that impurities outside of the reaction vessel may diffuse into the polycrystalline silicon inside of the reaction vessel.
In a reaction vessel made of quartz, quartz itself contains impurities in quantities of about 0.1 ppm, and is thereby a contamination source for polycrystalline silicon at high temperatures. Besides, in the case where polycrystalline silicon is produced by the fluidized-layer method, there is the possibility that silicon will be deposited and stuck onto the reaction vessel, whereby cracks are produced due to the difference between the thermal expansion coefficient deposited silicon and the reaction vessel.
In a reaction vessel made of SiC, there is the possibility that carbon will enter the polycrystalline silicon from the SiC to contaminate the polycrystalline silicon with carbon in the same manner as in the reaction vessel made of graphite. In addition, in order to enhance the mechanical strength of the reaction vessel, an outer cylinder made of metal is placed on the outside of the reaction vessel, which is heated from the outside of the outer cylinder in many cases. The present inventors have discovered that there is also the possibility that impurities contained in the outer cylinder can contaminate the polycrystalline silicon within the reaction vessel through the reaction vessel.